Radial flow electrofilter

ABSTRACT

A radial flow electrofilter having a plurality of parallel planar horizontal electrodes. The filter may be provided with backflush means, in which case the filter medium preferably consists of non-deformable particles, or else a removable filter pack, preferably employing polyurethane foam, may be employed. In operation, finely divided solids, contained in a liquid of low electrical conductivity, may be efficiently removed by passing the liquid radially through the filter, while subjected to a unidirectional current electric field, from the periphery of the filter toward its center.

FIELD OF THE INVENTION

This invention relates to electrofiltration apparatus and to processesfor electrofiltration employing such apparatus.

BACKGROUND OF THE INVENTION

It is known to remove solids from hydrocarbons and other organic liquidsby the use of electrofilters. The electrofilter is a device having afilter bed of a porous medium in which an electric field is maintainedby one or more energized electrodes. The electric field is of highintensity so that the solids are removed from the liquid stream bybecoming tenaciously attached to the surfaces of the porous medium. Suchporous medium may be, for example, a polyurethane foam or may becomposed of hard granular particles.

Among the electrofilters of this general type, mention may be made ofthose shown in U.S. Pat. No. 3,799,856 to Franse, U.S. Pat. No.3,891,528 to Griswold, U.S. Pat. No. 3,928,158 to Fritsche, U.S. Pat.No. 4,059,498 to Crissman et al and U.S. Pat. No. 4,040,926 to Oberton.

It is an object of this invention to provide improved electrofiltrationapparatus.

It is a further object of this invention to provide a removable filterpack for an electrofiltration apparatus.

It is still an additional object of this invention to provide anefficient electrofiltration process.

Other objects of the invention will become apparent from the followingdescription.

SUMMARY OF THE INVENTION

In accordance with this invention, there is provided an improvedelectrofilter for use in the removal of finely divided solids fromliquids of low electrical conductivity. The electrofilter comprises avertical cylindrical metallic vessel; a cylindrical porous bed of adielectric filtering medium disposed centrally in said vessel, spacedapart from the walls of said vessel, thereby providing an annular spacebetween the bed and the vessel walls; a plurality of parallel planarelectrodes extending horizontally through at least a major portion ofthe bed; fluid inlet means to the vessel fluidly communicating with theannular space; tubular fluid collector means extending axially throughat least a major portion of the porous bed; fluid outlet means fluidlycommunicating with the collector means; and conductor means forsupplying electrical potential to at least alternately spaced saidelectrodes to create electric fields between adjacent electrodes. Theelectrofilter is either provided with backflushing means or the porousbed of a dielectric filtering medium and the planar electrodes and thetubular collector means, together with conductor means are included in aself-contained filter pack (i.e., one having no separate containmentmeans), removably mounted centrally in the vessel.

In the preferred embodiments, an odd number of electrodes is employedand means are provided to electrically ground the topmost andbottom-most electrodes and the alternately spaced electrodes betweenthem. The conductor means for supplying electrical potential is arrangedto supply the remaining electrodes.

In one embodiment, the electrofilter of this invention includes alsobackflushing means, which may include a back-flush fluid inlet, abackflush fluid distributor in the bottom portion of the vessel fluidlycommunicating with the backflush inlet, and a backflush fluid outlet inthe upper portion of the vessel. Non-deformable particles such as glassor porcelain beads or particles of a silicon dioxide containing mineral,such as those disclosed in the above referred to Oberton patent, arepreferred as the dielectric filtering medium in this embodiment,although non-conductive deformable material having voids, such asopen-pore polyurethane foam or nylon mesh wrapped in layers or nyloncord wrapped to create voids, may also be employed. When non-deformableparticles are employed, they are disposed interiorly of a cylindricalfluid distributor.

In another embodiment, no provision is made for backflusing the filterbed, and the bed of dielectric filtering medium, the electrodes and thecentral collector means, together with suitable conductor means, areincorporated in a filter pack removably mounted centrally in the vessel.Non-conductive deformable material having voids, such as mentionedabove, is the preferred filter medium in this embodiment, open-porepolyurethane foam, preferably compacted between the electrodes, beingmost preferred. In this type of arrangement, it is preferred to employan inlet for the fluid to be treated which is tangential to the sidewallof the vessel, thereby being adapted to create a circular flow of fluidaround the outside of the filter pack. Alternately, however, otherdistributor means, such as multiple entry points and manifolds, may beemployed.

Another aspect of the invention is directed to the removable filterpack. This comprises a plurality of parallel annular planar electrodes;a central tubular collector extending through the center holes of theelectrodes; alternate electrodes being in grounding electrical contactwith the collector and the remaining electrodes having center holessufficiently larger than the diameter of the collector to provideelectrical clearance; conductor means for supplying electrical potentialto the remaining electrodes; and annular layers of dielectric filteringmaterial positioned between the electrodes.

Preferably, an odd number of electrodes is employed, the topmost andbottom-most electrodes and the alternately spaced electrodes betweenthem being those in grounding electrical contact with the collector. Thedielectric filter bed material is preferably a non-conductive deformablematerial having voids, such as mentioned above, most preferablyopen-pore polyurethane foam, sheets of this material preferably beingcompacted between the electrodes and the filter pack beingself-contained.

A further aspect of the invention relates to a process for theelectrofiltration of a liquid of low electrical conductivity containingfinely divided solids, comprising passing the liquid in a radialdirection through a cylindrical porous bed of a dielectric filteringmedium, the bed being traversed by a plurality of parallel planarelectrodes perpendicular to the axis of the bed and extending through atleast a major portion thereof, a unidirectional, preferably continuousdirect current, electric field being maintained between adjacentelectrodes; discontinuing the electric fields when the bed becomesloaded with finely divided solids to the extent that filteringefficiency becomes impaired; and removing these solids from the bed bybackflushing with a backflush liquid. The preferred direction of flow ofthe liquid being treated is from the outer periphery of the bed towardits center.

The bed in this process preferably consists of non-deformable particles,such as glass or porcelain beads or particles of a silicon dioxidecontaining mineral, and the electrodes are preferably permeable.

Another process embodiment of the invention is one wherein a liquid oflow electrical conductivity containing finely divided solids is passedtangentially into a cylindrical vessel containing a centrally positionedcylindrical filter pack spaced apart from the walls of the vessel,thereby affecting circular flow of the liquid around the periphery ofthe filter pack. The liquid is caused to pass in a radial directionthrough the filter pack, which comprises a self-contained bed of anon-conductive deformable material having voids, traversed by aplurality of parallel planar electrodes perpendicular to the axis of thebed and extending through at least a major portion thereof. The treatedliquid is collected in an axially positioned collector and thenwithdrawn from the vessel. A unidirectional, preferably continuousdirect current, electric field is maintained between adjacentelectrodes. The non-conductive deformable material is preferablypolyurethane foam, preferably compacted between the electrodes.

In both process embodiments, the preferred voltage gradient ranges from1 to 60 KV per inch and the preferred electrode spacing is from 1 to 5inches apart. It is also preferred in each embodiment to employ an oddnumber of electrodes wherein the topmost and bottom-most and alternateelectrodes between them are grounded and the remaining electrodes areenergized.

The present processes are well adapted to the treatment of organicliquids, for example, hydrocarbon liquids. In the treatment ofhydrocarbon liquids such as absorption oils and jet fuels, polyurethanefoam may advantageously be used as the filtering medium.

The radial flow of the liquid being treated from the outer perimeter ofthe filter medium to its center, as described above, has the advantageof having the dirty oil at the outer perimeter where the radial flow isthe slowest, as opposed to the center, where the flow rate is thegreatest. As the rate increases with flow to the center collector,cleaner oil is being further cleaned. Radial flow in the reversedirection, i.e., from the center outward, has the disadvantage that thedirtiest oil is treated at the highest rate of flow.

It will be seen, however, that regardless of the direction of the flow,the apparatus involved may in some cases be the same. In such cases(e.g., the embodiment shown in FIGS. 1 and 2, described below), althoughthe apparatus is described in the specification and claims employingterminology which assumes flow from the outside to the center, theapparatus would still be the same and within the intended scope of theclaims if the flow were in the other direction. Similarly, although theterminology in this specification assumes operation of the apparatus ina vertical position, the apparatus can be employed in other than avertical position since its operation in not gravity dependent.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The invention is illustrated by but not limited to the exemplaryembodiments described below.

Referring to the drawings:

FIG. 1 is a longitudinal vertical cross-section of one embodiment of anelectrofilter of this invention incorporating backflushing means.

FIG. 2 is a transverse horizontal cross-section taken along line 2--2 ofthe electrofilter shown in FIG. 1.

FIG. 3 is a vertical view, mainly in cross-section, of anotherelectrofilter of this invention, wherein a removable filter pack isemployed.

FIG. 4 is a transverse horizontal section taken along line 4--4 of theelectrofilter shown in FIG. 3.

FIGS. 1 and 2 show an electrofilter of this invention whereinbackflushing means are provided. This filter includes a generallycylindrical metallic electrofilter vessel 1 having a fluid feed inlet 2in the sidewall and provided with a metallic cover 3 having a centrallypositioned vertical neck portion 13. Inside the vessel 1, concentricwith and spaced a small distance apart from the vessel walls, is acylindrical distributor liner 4 having meter orifices 5, which may becovered by screens or slotted covers 6. Distributor liner 4 is fastenedat its top and bottom to the wall of vessel 1. Interiorly of thedistributor liner 4 is a porous bed of a dielectric filtering medium 7.In the embodiment shown in this Figure, the preferred filter medium isglass or porcelain beads, although other types of non-deformabledielectric particles may be employed, such as those disclosed in ObertonU.S. Pat. No. 4,040,926. The screens or slotted covers 6 serve to keepthese particles from plugging the orifices 5. Less preferred in thisembodiment are open-pore polyurethane foam, nylon and othernon-conductive materials having voids.

The distributor liner 4 and screens or slotted covers 6 may be metallicor non-metallic. The orifices 5 are located in that portion of thedistributor liner 4 opposite to what may be termed the main treatingzone, that is the zone between the topmost and bottom-most electrodes12. A tubular collector 8, preferably metallic, having meter orifices 9is centrally positioned within vessel 1 and extends most of the heightof the vessel. A tubular extension 10 of the collector 8, lacking themeter orifices 9, extends through the vessel cover 3 and serves as theproduct outlet. A plurality of planar electrodes 11 and 12 extendhorizontally through the porous bed 7. These electrodes are annular inshape with central holes large enough for the outlet collector 8 to passthrough. As shown in FIG. 1, the electrodes 11 and 12 are permeable,being formed of expanded metal or other open material. However, all theelectrodes except the topmost and bottom-most electrodes 12 may also beof solid construction, although these are less desirable because theywould present obstruction to backflushing, described below. As shown inFIG. 1, there are an odd number of electrodes, of which electrodes 11are at ground potential, and electrodes 12, alternating with electrodes11, and including the topmost and bottom-most electrodes, are energized.Electrodes 11 are in contact, mechanically and electrically, with outletcollector 8, which in the preferred embodiment is metallic and inelectrical communication with vessel 1 by way of the neck portion 13 ofcover 3. An annular space 14 exists between outlet collector 8 and thesidewall of neck portion 13. Energized electrodes 12, mechanically andelectrically separated from collector 8, are connected together as by ametal conductor, such as a wire or rod 15, and supported as a unit byentrance bushing and support 16, which, as shown here, extends centrallythrough the bottom of vessel 1. As shown in the drawing, electrodes 12are larger in diameter than electrodes 11 so that they can be connectedto conductor 15 while providing electrical clearance for groundedelectrodes 11. Alternatively, electrodes 11 and 12 may be made the samediameter and tabs provided on electrodes 12 to provide for the necessaryconnections and clearances. An electrical conductor 17 passing throughentrance bushing 16 connects outside vessel 1 with a direct currentpower supply, not shown, and inside vessel 1 with the energizedelectrode assembly (electrodes 12 and conductor 15).

As shown in the drawing, the top 3 of vessel 1 serves as an uppergrounded electrode and a backflush inlet header 18 and communicatingfluid distributor outlets 19, at the bottom of vessel 1, serve as bottomgrounded electrode.

Instead of the topmost and bottom-most electrodes 12 being energized,these and alternate electrodes can be made to serve as groundedelectrodes, with the remaining electrodes energized. Also, both sets ofelectrodes may be energized and separate power sources may be employedfor this purpose. Although an odd number of electrodes is preferred sothat the topmost and bottom-most electrodes are at the same potential,an even number may also be employed down to a minimum of two. The onlyessential is that an electric field may be maintained between adjacentelectrodes.

The fluid inlet header 18 for backflushing purposes extends through thebottom of vessel 1 and communicates fluidly with fluid distributoroutlets 19. These outlets are preferably slotted to prevent theparticles of the filter medium from plugging them. Suitable for thispurpose are Johnson Well Screens®. Annular space 14 serves as an outletannulus for the backflush fluid, being provided with an outlet conduit20. A pump out drain 21 is provided in the bottom portion of vessel 1.If the intermediate electrodes (all but the topmost and bottom-mostelectrodes 12) are of solid construction, the backflushing path will besuitably modified.

In operation, a raw liquid feed containing finely divided solidparticles is introduced into vessel 1 through feed inlet 2 and passesthrough the orifices 5 of the distributor 4 and then in a generallyradial direction through the filter medium 7 to the outlet collector 8.During its passage through the medium 7, the feed is subjected to theaction of a continuous direct current or other unidirectional currentelectric field between adjacent electrodes, the result of which is thatthe finely divided solid particles separate from the feed and areretained in the medium.

The liquid product of this operation passes through the orifices 9 ofthe collector 8 and upwardly through the collector and its tubularextension 10 which serves as the product outlet conduit.

When the filter medium 7 becomes loaded with the separated solids to theextent that the efficiency of the filter is impaired, the filter bed isbackflushed with a suitable flushing fluid, such as a raw feed stream orother stream which can be recycled back into processing or ortherwisedisposed of while containing the contaminant. This fluid is introducedthrough backflush inlet header 18 and distributor outlets 19 and passesupwardly through the filter medium 7 and the perforations of electrodes11 and 12, i.e., transversely through the electrodes, and into theannular space 14, from which it exits by means of backflush outletconduit 20.

The spacing between the electrodes and the voltage employed areinterrelated. In general, these are set so that a voltage gradient inthe range of 1 KV to 60 KV per inch is achieved, the latter figure beingabout the limit for commercial power supplies. It is possible to designand use equipment with higher potentials, but after 75 KV these arecostly. The potential gradient chosen will depend on the nature of thesolids in the feed. The electrodes are ordinarily spaced from 1" to 5"apart, as required to achieve the desired potential gradient, takinginto consideration the power supply capability.

FIGS. 3 and 4 illustrate an electrofilter of this invention employing aremovable filter pack and having no provision for backflushing. Thisfilter includes a generally cylindrical metallic electrofilter vessel101 having a fluid feed inlet 102 in its sidewall and provided with aremovable cover 112. Occupying most of the interior of vessel 101 is acentrally positioned removable cylindrical filter pack 103, resting on aflange 104 of a product outlet conduit 111, which passes through thebottom of the vessel. The filter pack 103 is secured to flange 104 by aquick disconnect device, not shown.

Filter pack 103 comprises a centrally positioned vertical metallic fluidoutlet collector 108, provided with orifices 115, alternating groundedand energized annular planar electrodes, 106 and 107, respectively,which may suitably be made of solid aluminum plate, and layers ofopen-pore polyurethane foam 105 between adjacent electrodes. The numberof such layers between adjacent electrodes is not indicated in thedrawing. The construction of the filter pack will be made clear by thefollowing description of a preferred procedure for assembling it.

The polyurethane foam layers are cut from sheets of foam in an annularshape. An electrode 106, next to flange 104, is securely fastened to theoutlet collector 108 through which it is grounded. One or more layers offoam 105 is inserted over the collector 108 and rests on the bottomelectrode 106. An annular electrode 107 with tab 116 for electricalconnection is then slipped over the collector 108 and positioned on thefoam layers 105, the central hole in the electrode being sufficientlylarger than the diameter of the collector 108 to provide for therequired electrical clearance. One or more additional layers of foam 105is inserted and another grounded electrode 106 is inserted overcollector 108. This continues up to the topmost grounded electrode 106awhich is secured to a stud 117 projecting from the top closed end ofcollector 108 by means of a nut 109. The central portion of electrode106a may be shaped as required for this purpose. Top electrode 106a ismade to compact and compress the entire pack against the bottomelectrode 106 by tightening nut 109. A cover 110 is provided over nut109 to prevent bypassing of fluid around the nut.

Collector 108 may be made of an insulating material such as polyvinylchloride instead of metal, but in that case, electrodes 106 and 106amust be electrically tied together and taken to a ground potential atthe vessel. Electrodes 107 in that case may touch collector 108 withoutshorting out.

An electrical entrance bushing 114 in a flanged housing 113 in the sidewall of vessel 101 provides the means of introducing an electricalconductor 118 into the vessel. Conductor 118 is connected to each of theenergized electrodes 107 at the tabs 116.

The operation of the embodiment of FIGS. 3 and 4 is generally similar tothat of FIGS. 1 and 2. A raw liquid feed containing finely dividedsolids is introduced into vessel 101 through feed inlet 102 and passesin a generally radial direction through the polyurethane layers 105 tothe outlet collector 108. During its passage through the polyurethanelayers 105, the feed is subjected to the action of a direct currentelectric field between electrodes, the result of which is that thefinely divided solids separate from the feed and are retained in thepolyurethane. The liquid product of this operation passes through theorifices 115 of the collector 108 and downwardly through the collectorto outlet conduit 111.

When the polyurethane layers 105 become loaded with separated solids tothe extent that the efficiency of the filter is impaired, the filterpack 103 may be removed and replaced. The removed pack 103 may bediscarded or cleaned and reused.

The comments regarding the spacing of the electrodes and the voltagesemployed made in connection with the filter of FIG. 1 apply to thefilter of FIG. 2 also.

A 30 gallons per minute filter pack for the filter of FIGS. 3 and 4 maysuitably be 20" in diameter and 24" deep, with 3" spacing betweenadjacent electrodes filled with 20 pore per inch polyurethane foam. Thecentral outlet collector 108 may suitably be formed from a 4" diameterpipe. The vessel housing such a filter pack may suitably have dimensionsof 30" O.D.×231/4" shell length. The filter is suitably powered by a 5KVA power supply and may be fitted with an automatically controlledoutlet valve to shut upon high amperage or power failure.

An electrofilter of this invention employing 20 pore per inch open-porepolyurethane foam was tested on absorption oil used in a refrigeratedabsorption plant for LPG recovery from pipeline gas. The absorption oilused in this process is contaminated by "pipeline dust". These solidscause corrosion and poor heat exchange efficiency. Evaluation studiesindicated that a 60-90% removal of these solids is possible using theelectrofilter of our invention.

In the evaluation studies, a module which was 1/60 of the 30 gpmelectrofilter referred to above (same diameter but 1/60 of the electrodedepth) was employed. This module duplicated the spacing, gradient andfluid flow anticipated for the 30 gpm filter. Four drums of contaminatedabsorption oil, two from one plant unit and two from another, were runthrough the module in once through flow at 1/2 gpm, the flow being fromthe outside radially toward the central 4" diameter collector. Thesamples from the first unit analyzed 25 mg./gal. solids and contained1.0 mg/gal. after treatment, a 96% removal. The samples from the otherunit analyzed 47 mg./gal. and contained 2.0 mg/gal. after treatment,likewise a 96% removal. The total loading on the filter element was 5.4grams after the four drums were treated.

The equipment was then set up for a continual recycle of the oil, withthe contaminants retained from two previous samples being graduallyadded to the oil. This was continued until the filter medium was loadedwith all available contaminants, estimated to be 10.7 grams total. Thevoltage was stable at 30 KV and 0 ma. The filter pack was removed fromthe system and washed to recover the solids from the medium. Theseweighed 10.2 grams. The foam weight was 61 grams. Other testing hasshown an allowable loading on foam to be 1 to 2 times its weight. Thefilter element can be cleaned easily outside the vessel. Thecontaminants wash off with a jet spray of clean absorption oil.

The following is a semi-quantitative spectrographic analysis of thecontaminant solids, which analyzed 75.30% ash.

                  TABLE 1                                                         ______________________________________                                        SEMI-QUANTITATIVE SPECTROGRAPHIC ANALYSIS                                                   Approximate %,                                                  Element       on Ashed Basis                                                  ______________________________________                                        Aluminum      0.2                                                             Barium        Trace                                                           Calcium       0.3                                                             Chromium      0.2                                                             Copper        0.5                                                             Iron          63                                                              Lead          0.2                                                             Magnesium     0.06                                                            Manganese     0.3                                                             Molybdenum    0.02                                                            Nickel        0.1                                                             Silicon       3                                                               Strontium     Trace                                                           Tin           Trace                                                           Titanium      0.2                                                             Zinc          0.5                                                             ______________________________________                                    

Test were also carried out using a 24" diameter by 12" highelectrofilter equipped with three parallel planar electrodes with layersof 20 pores per inch open-pore polyurethane foam compacted between them.The middle electrode was energized and the upper and lower electrodesgrounded by way of a central tubular collector. The liquid feed wasintroduced centrifugally into an annular space provided between the wallof the filter vessel and the periphery of the electrode-foam assembly.The treated liquid was withdrawn from an outlet at the bottom of thetubular collector. The feed was passed through the filter at the rate of25 gpm. Jet fuel from two different sources, artificially contaminatedwith a contaminant composed of 1 part Fe₂ O₃, 1 part fine Arizona RoadDust and 1 part of Air Filter Test Dust, were employed as feeds. Thefurther conditions and results of the tests are shown in the followingtable.

                                      TABLE 2                                     __________________________________________________________________________    Test     1   2    3   4   5   6   7   8                                       __________________________________________________________________________    Fuel     PR(a)                                                                             GOC(b)                                                                             GOC GOC GOC GOC GOC GOC                                     Temp., °F.                                                                      85  80   78  78  80  78  78  78                                      Inlet Solids                                                                  Mg/gal.  30  30   30  30-45                                                                             30  30  40  45                                      Packing                                                                       No. of layers(c)                                                                       20  24   20  24  1   1   16  24                                      Porosity, PPI                                                                          20  20   20  20  20  40  40  20                                      Grams    304 374  306 440 308 361 364 352                                     Grad. KV/Inch                                                                          30  30   30  60  60  60  60  60                                      Current, Ma.                                                                           0.45                                                                              0.28 0.07                                                                              0.2 0.02                                                                              1.2 0.02                                                                              0.25                                    Grams of Solids                                                               added before 1st                                                              visible residue                                                               in product                                                                             145 20   30  520 0   20  130 450                                     Equivalent gals.                                                              of fuel contain-                                                              ing 10 mg/gal.                                                                solids   14500                                                                             2000 3000                                                                              52000                                                                             0   2000                                                                              13000                                                                             45000                                   % Loading                                                                              48  5.4  9.8 120 0   5.6 36  125                                     __________________________________________________________________________     (a)Jet fuel in Petreco supply tank.                                           (b)Hydrofined jet fuel from Gulf Oil Co., Port Arthur                         (c)Between adjacent electrodes                                           

It will be seen from Table 2 that a multitude of foam layers betweenadjacent electrodes yields better results than a single thick sheet offoam, even when the voltage gradient is increased. Accordingly, the useof multiple layers is preferred. The weight of foam in place isindicative that the compression factors were comparable.

It will be evident that the foregoing description is illustrative of,rather than limitative upon, the invention as defined by the appendedclaims and that various changes and modifications can be made in theapparatus and methods exemplified without departing from the spirit ofthe invention.

We claim:
 1. An electrofilter for removal of finely divided solids fromliquids of low electrical conductivity comprising:(a) a verticalcylindrical metallic vessel; (b) cylindrical fluid distributor meansconcentric with and spaced apart from said vessel; (c) a porous bed of adielectric filtering medium disposed interiorly of said cylindricalfluid distributor means; (d) a plurality of annular parallel planarelectrodes extending horizontally through at least a major portion ofsaid bed, all said electrodes being permeable; (e) fluid inlet means tosaid vessel fluidly communicating with said annular space; (f) tubularfluid collector means extending axially through at least a major portionof said bed; (g) fluid outlet means fluidly communicating with saidcentral tubular collector means; and (h) conductor means for supplyingelectrical potential to at least alternately spaced said electrodes tocreate electric fields between adjacent electrodes;said electrofilterbeing provided with backflushing means adapted to cause backflushingfluid to pass transversely through the electrodes, said backflushingmeans comprising: (i) backflush inlet means; (j) backflush fluiddistributor means in the bottom portion of said vessel fluidlycommunicating with said backflush fluid inlet means; and (k) backflushfluid outlet means in the upper portion of said vessel.
 2. Anelectrofilter of claim 1 wherein an odd number of said parallel planarelectrodes is employed, comprising also means for electrically groundingthe topmost and bottom-most electrodes and the alternately spacedelectrodes between them, said conductor means (h) for supplyingelectrical potential to at least alternately spaced electrodes beingarranged to supply the remaining electrodes.
 3. An electrofilter ofclaim 1 wherein said dielectric filtering medium consists ofnon-deformable particles.
 4. The electrofilter of claim 3 wherein saidnon-deformable particles are glass or porcelain beads or particles of asilicon dioxide containing mineral.
 5. A process for theelectrofiltration of a liquid of electrical conductivity containingfinely divided solids comprising passing said liquid in a radialdirection through a cylindrical porous bed of a dielectric filteringmedium, said bed being traversed by a plurality of parallel planarpermeable electrodes perpendicular to the axis of said bed and extendingthrough at least a major portion thereof, a unidirectional currentelectric field being maintained between adjacent electrodes;discontinuing said electric fields when said bed becomes loaded withfinely divided solids to the extent that filtering efficiency becomesimpaired; and removing said finely divided solid from said bed bybackflushing it with a backflushing liquid, said backflushing liquidbeing passed transversely through the electrodes.
 6. The process ofclaim 5 wherein the direction of flow of said non-conductive liquidcontaining finely divided solid particles is from the outer periphery ofsaid bed toward its center.
 7. The process of claim 6 wherein said bedconsists of non-deformable particles.
 8. The process of claim 7 whereinsaid non-deformable particles are glass or porcelain beads or particlesof a silicon dioxide containing mineral.
 9. The process of claim 6wherein said unidirectional current electric field is a continuousdirect current field.
 10. The process of claim 9 wherein a voltagegradient between 1 and 60 KV is employed and the electrodes are spacedfrom 1 to 5 inches apart.
 11. The process of claim 6 wherein an oddnumber of electrodes are employed.
 12. The process of claim 11 whereinthe topmost and bottom-most electrodes and the alternate electrodesbetween them are at ground potential and the remaining electrodes areenergized.